Thanks for getting the discussion started, Denny. My intent with spending all the time working on this article was largely to start a conversation on finding ways to predict and target beer balance. I'm glad the conversation is starting.
Ultimately, the best way to make the beer you want is to brew it once then make corrections in pursuit of your intent, but I'm always interested in methods to frontload that effort with calculations (the fewer repetitions needed to perfect a recipe the better).
The Mad Fermentationist (not me, I'm the Mad Alchemist!) emailed me and proposed that we start thinking along the lines of the residual extract (which I believe is the same thing as real extract, so that's what I ran with).
To push this discussion further along, here's some thoughts I sent him about that approach.
I added columns for Real Extract in both Plato and Specific Gravity to the far right of the data chart.https://docs.google.com/spreadsheet/ccc?key=0Ai1Yv492QZYUdFN1YWpYZTFxUm1reWN2WEx2a0xpUkE
I originally used RDF (Real Attenuation) instead of ADF (Apparent Attenuation) for similar reasons. Ultimately, since I decided to adjust the Balance according to a beer relative to all other beers, using ADF vs. RDF became inconsequential (because your beer's RDF in comparison to the average is the same as your beer's ADF in comparison to the average). I switched back to ADF so people didn't have to bother with RDF = ADF * 0.8192.
Real Extract is ultimately a similar approach, though might be more accurate in terms of perceived sweetness of the final beer.
If we were to decide to create a new number scheme from BU:GU entirely, I think it might yield some useful results (part of my decision to go with what I did for RBR was to make it more accessible since many people are familiar with BU:GU already, and the numbers are essentially the same).
Anyway, I also dropped in a thought on a potential method to calculate this under "Perceived Bitterness?" in the last column.
It is IBU/RE (°P)
International Bitterness Units / Real Extract (degrees Plato)
RE = (0.1808 × °Pinitial) + (0.8192 × °Pfinal)
Interestingly, this (IBU/RE (°P)) gives numbers roughly on a scale from 1 to 10, with the outlier being Imperial IPA at 13.4. It's actually the only beer style above 10 using this equation.
I went ahead and colored the columns as well, which allows us to see that beers relative to one another look roughly the same as BU:GU and RBR.